Can you kindly share your store URL so that I can check it further.
The image you are using is too small, you can look at the "Slider image data" fields to see they are 218 and 312 px.
You should find a better background image with better size and high quality. I suggest you watch this video from Shopify help Center, in which they suggest the best practices to select image sizes for your slideshow:
Thanks for your response. Although I can see why you think this is the problem, this isn't it.
I'll show you a better example.
The image that I am uploading for the slider background is more than big enough to cover the dimensions.
The slider image is the smaller image which is fine. The theme just isn't showing the background image that I am uploading. No matter how big the image size is.
You are using custom theme.
So please Contact with that theme owner.
Otherwise we will check your issue.
But you have to pay for this.